1. Field of the Invention
The present invention generally relates to a surface acoustic wave device and a method of manufacturing the surface acoustic wave device, and more particularly, to a method of manufacturing a surface acoustic wave device that includes a joined substrate.
2. Description of the Related Art
Surface acoustic wave (hereinafter referred to simply as SAW) devices are being widely used today as bandpass filters in communication devices such as portable telephones. In this trend, there has been an increasing demand for SAW filters with higher performances, as portable telephones has been becoming more and more sophisticated.
SAW devices, however, have a problem of passbands that shift with temperature. To solve this problem and develop SAW devices having excellent stability with temperature has been a target to achieve in this field of technology in recent years.
Materials for SAW devices are piezoelectric materials such as lithium tantalate (hereinafter referred to simply as LT), lithium niobate (hereinafter referred to simply as LN), and crystal. Among these materials, LT and LN, which are being widely used today, have large electromechanical coupling coefficients that are advantageous in achieving broadband filter characteristics. However, these materials have a drawback of having poor stability with temperature.
On the other hand, crystal is a piezoelectric material that has excellent stability with temperature, but has a drawback of having a small electromechanical coupling coefficient.
So as to improve the above characteristics and obtain a piezoelectric material with a large electromechanical coupling coefficient and excellent stability with temperature, various methods have been suggested. Yamanouchi, et al., for example, suggest a substrate produced by forming a silicon oxide film having an opposite temperature coefficient on the surface of an LN or LT substrate (“IEEE Trans. on Sonics and Ultrasonics”, vol. SU-31, pp. 51-57, 1984, hereinafter referred to as “Non-Patent Document 1”). Nakamura, et al. suggest a method of improving the stability with temperature changes by utilizing the field short-circuiting effect of a polarization inversion layer that is thinner than the SAW wavelength formed on the surface of a LT substrate (Japanese Patent Publication No. 2516817, hereinafter referred to as “Patent Document 1”). Onishi, et al. suggest a method of increasing the stability with temperature by joining a thin piezoelectric substrate and a thick low-thermal-expansion material substrate directly to each other so as to restrict expansion and contraction of the piezoelectric substrate with changes in temperature (Japanese Unexamined Patent Publication No. 11-55070, hereinafter referred to as “Patent Document 2”, and “Proc. of IEEE Ultrasonics Symposium”, pp. 335-338, 1998, hereinafter referred to as “Non-Patent Document 2”). Yamanouchi, et al. also suggest the same structure that is formed using an adhesive or the like (“Proc. of IEEE Ultrasonics Symposium”, pp. 239-242, 1999, hereinafter referred to as “Non-Patent Document 3”). Further, Isokami suggests a method of improving SAW characteristics by joining two substrates of different types through a solid phase reaction (Japanese Unexamined Patent Publication No. 9-208399, hereinafter referred to as “Patent Document 3”).
With the substrate produced by forming a silicon oxide on the surface of a LT or LN substrate disclosed in Non-Patent Document 1, however, it is difficult to maintain the film quality and thickness of the silicon oxide film at a certain level, and frequency fluctuations cannot be avoided in the SAW device. Furthermore, a silicon oxide film is also formed on comb-like electrodes (interdigital transducers, IDTs) in this conventional method. As a result, a SAW propagation loss becomes larger, and this method can be applied to only limited types of devices.
With the method of forming a polarization inversion layer disclosed in Patent Document 1, it is difficult to control the depth of the polarization inversion layer that greatly affects the characteristics of each SAW device.
With the methods of joining a low-thermal-expansion material substrate to a piezoelectric substrate disclosed in Patent Document 2 and Non-Patent Document 2, it is necessary to employ a low-thermal-expansion material substrate having a low Young's modulus such as glass, so that the wafer does not break during a high-temperature annealing process for joining substrates that have been cleaned. As a result, distortion caused by the difference between the thermal expansion coefficients of the two joined substrates does not sufficiently reach the piezoelectric substrate, and the temperature characteristics are not improved adequately.
With the method of joining a piezoelectric substrate and a low-thermal-expansion material at ordinary temperatures using an ultraviolet curing adhesive disclosed in Non-Patent Document 3, stress is eased by the adhesive at the interface, and the effect of improving the temperature characteristics is reduced accordingly.
With the method of using a substrate formed by joining through a solid phase reaction disclosed in Patent Document 3, it is necessary to carry out a heating process at such a temperature that does not cause melting after the substrate joining process. However, as the heating is performed at a temperature higher than 1000° C., a change is caused in the constant of piezoelectric substrate. As a result, an acoustic velocity change and frequency fluctuations are caused.